Along with the progress of high-power lasers, applications to industrial processing such as welding using laser light are widely employed. Especially in recent years, high-power fiber lasers having an output of as high as 10 kW have been developed and are expected to be used for medical and industrial applications. In such a high-power fiber laser, for example as illustrated in Non-Patent Literature 1, the core area is enlarged in a short optical fiber of several meters or less, thereby relaxing an output power limit due to nonlinearity. Moreover, in laser processing, the beam quality of emitted light greatly affects the processing efficiency. Since the beam quality strongly depends on a mode state of emitted light, an optical fiber capable of single mode transmission is used in a fiber laser.
Furthermore as illustrated in Non-Patent Literature 2, an optical fiber is coupled to an emitting end of the high-power laser described above, which is applied also to welding processing from a remote place. In this case, the beam quality at the emitting end is affected by an excitation state of a higher order mode in the coupled optical fiber. Therefore, connecting a multimode optical fiber having a large core area as a transmission optical fiber enables transmission of high-power light such as several kilowatts for several tens of meters or more, however, the beam quality at the emitting end becomes low. In order to enhance the beam quality, it is necessary to reduce the number of propagation modes; however, in structure design of the optical fibers in general, reduction of the number of propagation modes and enlargement of a core area are in a trade-off relationship, and thus an attempt to enhance the beam quality results in limiting the power that can be transmitted.
Furthermore as illustrated in Non-Patent Document 3, it is known that, by using a photonic crystal fiber having a hole structure, the trade-off between a single mode operation region and enlargement of a core area can be relaxed as compared with a general optical fiber in which a refractive index distribution is formed by adding a dopant to a core. Therefore, it is known that, in a photonic crystal fiber for a communication application, deterioration of tradition characteristics due to a nonlinear effect can be mitigated in an optical communication system. Furthermore in Patent Literature 1, it is known that the trade-off between the single mode operation region and enlargement of a core area can be further relaxed as compared with a uniform structure by using a photonic crystal fiber in which holes are arranged non-uniformly.